shrinkwrap ("decal") around entire object?
It is completely unclear to me what you are trying to accomplish, or why you want to use the tools proposed in the question. So I'll post an answer as blindly as trying to break a piñata:



(photo: AlejandroLinaresGarcia, Wikipedia)

If you want a cube with a texture, use the texture directly on the cube using UV coordinates:



If you need the cube to have different front and back, the images mapped have to correspond with those views:



To have some geometry converted to the shape of another, then you'd use the shrinkwrap modifier. Have the brain object be larger than the cube, the you can shrinkrwrap it around the cube using nearest surface point:



Brain model by mahesh downloaded from  http://www.blendswap.com (login required)